1. Field of the Invention
The present invention relates to a chemically amplified positive resist composition used for a lithography in microfabrication of a semiconductor, a photomask blank, and the like, and to a resist patterning process.
2. Description of the Related Art
It is well known that as LSI progresses toward a higher integration and a further acceleration in speed, a miniaturization of a pattern rule is required. Accompanying with this trend, an exposure method and a resist material are drastically changing. In particular, when a lithography of a pattern with 0.2 μm or less is carried out, KrF and ArF excimer laser beams, an electron beam, or the like are used as an exposure light source, and a chemically amplified type showing a good sensitivity to those high energy beams and giving a high resolution is used for a photoresist.
At the present, in a chemically amplified positive resist composition material whose solubility to an aqueous alkaline developer is suppressed by protecting (blocking) a phenolic hydroxy group with an acid labile group, a material mainly including a base resin having an acetal protecting group is generally used. In particular, a substance obtained by reacting a part of the hydroxy group of polyhydroxy styrene with a vinyl ether, which is excellent in a dry-etching resistance, is usually used. And in addition, a reactivity of the acetal protecting group with an acid is excessively high so that the acetal group can be easily removed, thereby leading to a high sensitivity and a high resolution. In order to render an etching resistance further, such a measure as an introduction of an aromatic unit having a condensed ring structure has been taken (for example, Japanese Patent Laid-Open (kokai) No. 2008-95009).
The foregoing base resin for the chemically amplified positive resist composition is used mainly for a KrF exposure resist, and after a lithography by the state-of-the art UV beam is shifted to ArF, it has still been used as a base resin for an electron beam exposure and an extreme ultraviolet beam exposure. For example, as disclosed in Japanese Patent Laid-Open (kokai) No. 2008-95009, a fine pattern with an excellent form having the minimum line width of 80 nm by using the resist film thickness of 240 nm could be successfully formed by a pattern exposure with an electron beam.
Meanwhile, a miniaturization of LSI is further progressed in recent years so that a process dimension is about to cut below 50 nm as the minimum line width. When the process dimension becomes so fine, the resist film thickness needs to be made 100 nm or less in a certain case depending on a surface material of a substrate to be processed in view of such factors as a structural strength to keep its pattern against a developer's surface tension, an adhesion strength to the substrate, and the like.
To address a part of the dry etching resistance problem of a resist film in a dry etching process of such a thin resist film, a method that an etching contrast can be obtained even if a thin resist film is used, such as a multilayer resist method in the case of a semiconductor device processing (for example, Japanese Patent Laid-Open (kokai) No. 2008-96684) and an etching mask film in the case of a photomask processing (for example, Japanese Patent Laid-Open (kokai) No. 2007-241060) are proposed.
As mentioned above, a part of the etching resistance problem is solved by using an etching auxiliary film in the lithography method by a resist film with a film thickness of 100 nm or less, but it became clear that the requirement is not necessarily satisfied in terms of compatibility with a high resolution of the resist film to be used.
It became newly apparent that, when a resist composition conventionally used to form a chemically amplified resist film having a high resolution is attempted to apply to a thin film only by dilution to form a fine pattern with the film thickness of 100 nm or less, a form at the resist pattern edge, so-called line edge roughness (hereinafter, referred to as LER), is drastically deteriorated to be improper.
For example, when a base resin having an acetal protecting group with a high resolution and a high sensitivity was used as the chemically amplified positive resist composition, the problem of drastic deterioration of LER appeared when the film thickness of the chemically amplified resist film became 100 nm or less, even though there was no significant problem of the LER deterioration when the film thickness was 150 nm.